Flyback switcher for electric vehicle safety designs

Technology News |
By Nick Flaherty

Power Integrations has launched an automotive-qualified flyback switcher with an integrated 750V MOSFET and secondary-side sensing for traction inverters, OBC (on-board charger), EMS (energy management DC-DC bus converters) and BMS (battery management systems) to provide an alternative power supply for an electric vehicle if the 12V battery fails.

“One important thing for the traction inverter is the functional safety, to be able to safe to use even in the presence of failure,” said Edward Ong, product marketing manager at Power Integrations.

The 12V battery is used to provide the gate drive voltage in the traction inverter so in the event that the 12V battery fails or is disconnected there is no 12V input to the gate drivers and the traction controllers. The functional safety requirements of the ISO26262 standared require an emergency power supply to provide the output voltage for the inverter controller system and the gate drivers to move the vehicle safely.

“This will be taking power from the HV bus which can be a wide range, as low as 30V and as high as 550V,” he said. “This is something that is really new.”

The InnoSwitch3-AQ flyback switcher uses Power Integrations’ high-speed FluxLink coupling to achieve ±3 percent accuracy for combined line and load regulation while eliminating both dedicated isolated transformer sense-windings and optocouplers. This high speed link allows the controller to maintains output voltage regulation even under the transient stress test, which is particularly challenging for PSR-based implementations.

“You need a feedback loop on the secondary side – in automotive optoisolators can’t be used as they degrade with time and this impacts the life of the power supply system so the dilemma is that you need the isolation to feed the data back to primary side,” said Ong.  

“But the FluxLink generates a magnetic coupling without the use of optoisolators,” he said. “The advantage of inductive coupling is that with magnetic coupling allows a faster response time so if at low traction the power supply can easily respond – the bandwidth of the feedback loop can easily go up to more than 20kHz,” he added.   

“Right now for a consumer vehicle the 400V bus can go up to 550V so the AQ is rated for 750V right now and it can scale up to emergency power supply for 800V bus,” he said.

The integrated 750 V MOSFET in the flyback switcher meets stringent automotive de-rating requirements, and an on-chip synchronous rectifier controller delivers above 90 percent efficiency at the nominal 400 VDC input voltage with less than 10 mW no-load energy usage across the input voltage range.

The InnoSwitch3-AQ family ICs are packaged in a surface-mount InSOP with 11 mm primary to secondary creepage, which exceeds the stringent requirements for high altitude (5000 m) isolation.

The DER-840Q reference design incorporates the InnoSwitch3-AQ and demonstrates start-up, shutdown and efficient operation from 30 VDC to 550 VDC input, as well as fast transient response and a variety of safety and protection features. A ‘StackFET’ can be added alongside the AQ chiip to support 800V designs, simplifying the bill of materials across a range of vehicles. 

The InnoSwitch3-AQ is available in the InSOP-24D package. Devices are available now with prices starting at $2.75 in 10,000-piece quantities

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